Receipt of the Ruth L. Kirschstein National Research Service Award would facilitate the applicant's long-term objective to become a clinician scientist and independent investigator within a pediatric surgical discipline. Erica M. Fallon, MD will focus on studying the basic mechanisms of pediatric liver injury, with particular attention to parenteral nutrition associated liver disease (PNALD). The proposed training plan will allow her to develop expertise in the biochemical techniques and understanding of lipid biochemistry and metabolism. She will obtain expertise in murine models of liver injury and steatosis, and acquire advanced skills in experimental design, data interpretation, and manuscript preparation. These goals will be accomplished within the Vascular Biology Laboratory at Children's Hospital Boston, and through research mentorship with co-sponsors Mark Puder, MD, PhD, Bruce Bistrian, MD, PhD, and Steven Freedman, MD, PhD. Parenteral nutrition (PN) is a life-saving therapy for patients unable to absorb enteral nutrients secondary to insufficient intestinal length or function. Before the development of PN, patients with insufficient gastrointestinal absorptive function commonly died of starvation and subsequent complications of malnutrition. Today, more than 30,000 patients in the US are permanently dependent on PN for survival. In children, the most serious long-term complication of intravenous nutrition, PNALD, results in a mortality rate approaching 90% within one year of diagnosis if infants are unable to be weaned off PN or fail to receive a liver / small bowel transplant. Recent evidence suggests that PNALD may result from the lipid emulsion component containing soy oils, which have large quantities of pro-inflammatory omega-6 fatty acids and hepatotoxic phytosterols. The Vascular Biology Laboratory has shown that a fish-oil based lipid emulsion prevents liver injury in mice and is efficacious in the treatment of PNALD in children. In order to determine the requirement and optimal ratio of essential fatty acids, in a soy-free or fish oil-based lipid emulsion, Dr. Fallon proposes a series of experiments to explore the essentiality of arachidonic acid (AA) and docosahexaenoic acid (DHA). In the proposed project, it is hypothesized that AA and DHA as the sole fatty acids will prevent EFAD. Using a murine model of EFAD, mice will be treated with DHA and AA at doses above and below amounts provided in the fish oil emulsion but without the accompanying linoleic (omega-6) and alpha-linolenic (omega-3) acids. Doses at which animals do not develop EFAD will be used in long term experiments. Next, studies will be undertaken to determine the optimal ratio of DHA to AA to prevent EFAD, and the metabolic pathways involved in the development of hepatosteatosis. This information will set the groundwork for the creation of non-toxic lipid emulsions for infants and children.